This invention relates to a novel method for quenching metal employing as the quenching medium aqueous solutions of water-soluble salts of certain copolymers of unsaturated dibasic acids and long chain alpha olefins.
The physical properties of metals, such as steel, can be modified by heat treatment, which generally involves heating the metal to elevated temperatures, followed by quenching in air, a molten salt bath, water, an aqueous solution of a water-soluble salt or a polyol, or oil. In such heat treatment, the rate of cooling is most important in obtaining the particular physical properties which are sought.
Water, for example, causes very rapid cooling of the metal, and with some metals, such as steel, may produce excessive strains which warp and crack the steel. On the other hand, hydrocarbon oils provide a relatively slow rate of cooling. Such slow cooling may provide a steel with desired ductility at the expense of hardness.
Aqueous solutions of various water-soluble polymers have been suggested for use as quenching fluids to provide cooling rates intermediate to and including those provided by water and hydrocarbon oils. The decrease in cooling rate provided by such solutions is believed to be due to various phenomena. For example, certain water-soluble high molecular weight polyalkylene glycols are believed to cause a reduction in cooling rate by coming out of solution at elevated temperatures and forming a higher-boiling insulating layer on the metal being quenched. Unfortunately, such glycols, like oil, have the disadvantage of producing stained or darkened metal parts due to drag-out on the hot parts.
Aqueous solutions of certain water-soluble salts of polyacrylic acid have also been suggested for use as quenching baths for steel and other metals. Such salts are believed to cause the formation of a relatively stable vapor envelope about the metal being quenched, which envelope substantially reduces the cooling rate. By use of such quenching baths, it is possible to obtain non-martensitic structures in steel without subsequent heat treatments.
A disadvantage of polyacrylate quenching baths is that, although the cooling rate can be decreased by increasing the concentration of the polyacrylate, such increase in concentration also causes an increase in bath viscosity. At high bath viscosities, some of the polyacrylate may be removed from the bath as a coating on the quenched metal. Such "drag-out" results in unstable quenching conditions, since bath concentration decreases with use. Thus, the bath must be monitored continually in order to maintain the desired quenching conditions, particularly bath concentration.
It is a primary object of the present invention to provide an aqueous medium for quenching metal whose composition can be varied to provide a broad range of quenching rates between the quenching rates of water and oil, as well as rates similar to those of oil.
Another object of this invention is to provide a novel method for quenching heated metal to obtain quenched metal parts having the desired physical properties and improved appearance.
A further object of this invention is to provide a new and useful process for cooling austenitized ferrous metal parts to produce therein non-martensitic or martensitic microstructures as desired.
Yet another object of this invention is to provide a novel quenching method using an aqueous bath comprising a solution of a water-soluble salt of certain copolymers, the viscosity of which solution does not vary significantly with copolymer concentration, whereby maintaining of desired quenching conditions is simplified.
These and other objects of this invention will become further apparent from a consideration of this specification, appended claims, and drawings in which: